Event stream processing (ESP) is a set of technologies designed to assist the construction of event-driven information systems. ESP technologies may include event visualization, event databases, event-driven middleware, and event processing languages, or complex event processing (CEP). ESP deals with the task of processing multiple streams of event data with the goal of identifying the meaningful events within those streams. ESP enables applications such as algorithmic trading in financial services, radio frequency identification (RFID) event processing applications, fraud detection, process monitoring, and location-based services in mobile devices. Within an ESP system a continuous data stream (comprising multiple, consecutive data items) is pushed through a query. Results of the query are subsequently pushed out of the system. Queries in ESP systems can be decomposed into a network of operators, each operator representing an atomic processing block. The operator network forms a directed, acyclic graph (DAG). Vertices of the DAG describe operators, while edges indicate the data flow. The total throughput, or the number of data items processed within a unit of time, is limited by the slowest operator in the DAG. This can lead to some operators being over-utilized while others are under-utilized, leading to suboptimal usage of the underlying system resources.